As you can see, I think a watermark does nothing. In just a couple of minutes, I removed his from the bottom corner and replaced it with "John Doe". I also replaced the origional watermark to keep Mark from getting mad at me!
My point is it is really not necessary to watermark an image. Anyone can remove it. It is still copyright material. If anyone can prove that they are the author (a high-res version is proof), they can go after damages if someone infringes on it.
I am really not worried about pictures we post here. They are too small to be useful for anything other than web use. They are worthless for print.
If some day, you are flipping through a magazine and see an ad with your photograph used without permission, GO GET 'EM!
You can buy/sell photographs here: [ulr]http://www.eyewire.com[/ulr]
Look how they protect them.